1. Field of the Invention
This invention relates to the field of draw forming material, particularly to the art of forming tubes and hollow shafts having wall thicknesses that vary along the length.
2. Description of the Prior Art
Various cold-drawing techniques for producing tubing and hollow shafts having a wall thickness that is either constant throughout its length or that varies along its length are known. These techniques conventionally use an external reducing die for forming the outer surface of the tube, and an internal mandrel for forming the inner surface and establishing the wall thickness in an annular space located between the die""s inner surface and the mandrel""s outer surface. U.S. Pat. Nos. 2,228,301 and 4,616,500 describe use of a tension force to draw a workpiece through a reducing die and over a mandrel. This technique works well on large diameter tubing, wherein a removable plug is used to transmit a large pulling or tension force to draw the tube through a die. U.S. Pat. Nos. 4,454,745; 4,487,357; 5,241,848 and 6,439,672 describe use of a compression force to push a mandrel through a workpiece and die.
Tension and compression drawing forces each can cause failure of a workpiece. When the outside diameter of the workpiece is drawn to a small diameter, the tension force that draws the workpiece through a reducing die may cause a tension failure at the small diameter section if the strength of the cross section is insufficient to carry the tensile load. A compression force can cause failure in the region of the die entry opening and the large diameter, where a thin wall is susceptible to local compression instability failure. A method is required to avoid tension and compression failures in a workpiece while drawing tubes, particularly tubes having both large and small diameter length portions.
An aluminum baseball bat is an example of a cold drawn product having a long transition section between large and small diameter zones. The barrel having a diameter of about 2.75 inches, a transition section, and handle having a diameter of about 0.75 inches are drawn from a starting tube having a constant outside diameter and uniform wall thickness using various dies. Variations in the length of the tubes after drawing, which may be about 0.50 inches in a nominal 36.0 inch long tube, are caused by variations in die lubrication, frictional contact of the workpiece with the die, and other process variables. To produce tubes having substantially the same length and smooth surfaces at opposite ends of the transition zone, there is a need to closely control the application of a tension force applied to the part after it is drawn through the reducing die. Unfortunately, it is difficult to control the actuating cylinder that applies the large drawing force with sufficient accuracy so that it also can apply the low magnitude of tension force that would produce smooth transition radii and an accurate overall length of the formed part. There is need to correct this deficiency in conventional equipment used for this purpose.
A tubular workpiece suited for draw forming may be either a seamless tube or a tube rolled from flat stock and having a welded seam running along its length. The seam is often stress relieved after welding to remove residual stresses. Before drawing a welded tube using a mandrel, excess weld material or scarf must be removed from the inner surface. Otherwise the weld material passing over the outer surface of the mandrel under high contact load impairs the drawing operation and requires a larger drawing force than would be necessary if the inner surface were smooth and flush. The increased drawing force increases the possibility of coring the mandrel in the area of the weld.
U.S. Pat. No. 6,311,703 describes an apparatus for scarf flushing or cleaning the inside of welded tubes by directing a high-pressure, high-volume stream of fluid onto the inner surface along the weld seam. The fluid stream removes the extraneous weld material and flushes it out of the tube. However, where the tube length is long and its diameter small, use of this apparatus and the related technique increases the time, difficulty and expense required to draw welded tubes.
An intrusion beam of the type used to strengthen a door of an automotive vehicle against collision damage and occupant injury is an example of a part fabricated from a tubular workpiece that may be either a seamless tube or welded tube. Preferably tubes for intrusion beams would have relatively thin walls in the vicinity of the ends and thicker walls along the tube length between the ends. The thicker wall region increases the structural strength and stiffness of the door beam in the area where the greatest bending stresses would be produced due to impact from a collision. Currently tubes for intrusion beams have a uniform diameter with a constant wall thickness.
It is preferable that a tube drawing method applicable to both unwelded and welded tube be used without changing the process to accommodate the surface condition of the tube and without incurring unnecessary complexity or cost. It is preferable also that variable wall thickness tubes can be drawn by such an improved process or method.
It is an advantage of this invention that structural failure of a workpiece due to tensile or compressive drawing forces is avoided, particularly while forming tubes having both large and small diameter sections without the use of mandrels.
It is another advantage of this invention that a drawn tube can be produced having relatively thin walls at its ends and thicker walls along a portion of the length between the ends. In the case where a tube having these characteristics is fabricated into a door intrusion beam for a motor vehicle, the resulting advantages include minimized vehicle weight, improved vehicle fuel economy, optimized strength of the passenger compartment door assembly, and enhanced passenger safety.
It is another advantage that a tube having a welded seam on its inner surface be draw-formed without having first to remove the rough inner seam by scarf removal and flushing or other techniques required by conventional methods before passing the workpiece over a forming mandrel.
It is yet another advantage that tubes having walls of differing thickness, constantly changing thickness, or short transition lengths of changing thickness located between zones of uniform thickness can be draw-formed regardless of the surface condition of the inner surface of the workpiece.
It is still another advantage of this invention that outer surface roughness and other discontinuities located near the ends of a tapered transition are removed easily and reliably and replaced by smooth blend radii.
In realizing these advantages, a method for forming from a tubular workpiece a tube or hollow shaft having a wall thickness that varies along its length, includes the steps of positioning relative to the workpiece a reducing die for reducing the outer diameter of the workpiece and establishing the outside diameter of the formed tube, advancing the workpiece through the die by applying to the workpiece a pulling force at a first location and a pushing force at a second location, the first and second locations being on opposite axial sides of the die, and changing the wall thickness of the tube as the workpiece advances through the die by changing the relative magnitudes of the pushing force and pulling force.
An apparatus for forming from a tubular workpiece a tube having a wall thickness that varies along the length of the tube includes a reducing die for reducing the outer diameter of the workpiece and contouring the outer surface of the tube being formed, a first actuating cylinder for producing a pulling force applied to the workpiece at a first location tending to advance the workpiece through the die, and a second actuating cylinder for producing a pushing force applied to the workpiece at a second location tending to advance the workpiece through the die, the first and second locations being on opposite axial sides of the die.